It is sometimes necessary or desirable for a physician to determine the concentration of gases in blood. This can be accomplished utilizing an optical sensor which contains an optical indicator responsive to the constituent of interest. The optical sensor is exposed to the blood, and excitation light is provided to the sensor so that the optical indicator can provide an optical signal indicative of a characteristic of the constituent of interest. For example, the optical indicator may fluoresce and provide a fluorescent optical signal as described in Lubbers et al U.S. Pat. No. RE31,897 or it may function on the principals of light absorbance as described, for example, in Fostick U.S. Pat. No. 4,041,932.
The optical sensor may include a gas permeable polymeric matrix with the optical indicator dispersed in the matrix. For accuracy of measurement, it is important to have a strong optical signal. The intensity of the optical signal is a function of the quantity of the optical indicator present in the polymeric matrix which in turn depends upon the quantity of the optical indicator that can be dissolved in the polymeric matrix during manufacture of the sensor.
Unfortunately, the preferred optical indicators are not very soluble in silicone polymers, the polymers of choice, and so less than the desired quantity of the indicator is present in the matrix. For example, decacyclene is a preferred fluorescent indicator for oxygen because it has maximum sensitivity in the appropriate wavelength range. However, decacyclene is not very soluble in silicone polymers. Silicone polymers are the most suitable oxygen permeable matrix materials known to be presently acceptable for use with blood. Accordingly, the problem cannot be solved by selecting another polymeric matrix material.
It has been suggested by others that the solubility of decacyclene in a silicone polymer might be increased by utilizing the hexa-t-butyl derivative of decacyclene. However, contrary to this suggestion, we have found that the use of hexa-t-butyl decacyclene as a substitute for decacyclene does not, in fact, yield significant and reproducible increases in the solubility in the silicone polymer.